Rivers carry a cocktail of substances, mostly invisible to the naked eye, that tells a story about its history and the land that it flows through. By analyzing the subtle differences in these substances across a watershed, aquatic scientists can decipher clues about the biological processes taking place on land and in the water itself. In our study of the headwaters of the Mississippi River, we untangled the clues of these processes to learn that human impacts like land conversion to agriculture and dam construction have subtle impacts on the chemistry and ecology of the river.
The Upper Mississippi River, a watershed that covers most of Minnesota and part of northwestern Wisconsin, drains a diverse range of land. The basin experiences a transition from predominantly agricultural land in the west to forests and wetlands in the east. In the agriculture-dominated Minnesota River, significant soil erosion causes high sediment concentrations and a muddy appearance. In stark contrast, the extensive wetlands of the Chippewa River in Wisconsin result in tea-colored water created by dissolved plant material.
Chemical Makeup Of Mississippi River Sediments
Our study focused on multiple forms of carbon carried by the Mississippi River, from dissolved carbon from plants and minerals to gaseous carbon dioxide to carbon carried by sediments. The most abundant form, dissolved inorganic carbon (DIC), comes from dissolving minerals in rocks and soils, as well as dissolved carbon dioxide from the atmosphere and carbon dioxide produced by biological respiration. The second-most abundant form of carbon is dissolved organic carbon (DOC), which is produced by the dissolution of organic material from plants, soil, microbes, and plankton. Finally, particulate organic carbon (POC) is organic debris from those same sources that have become attached to sediment particles. The different roles these types of carbon play in the river ecosystem lead to distinct changes in their chemical make-up over time.
The chemical make-up of these forms of carbon can be altered by certain physical and biological processes. Our study looked at three main properties of carbon. First, different organic source materials (such as plants and bacteria) contain different proportions of building block molecules. By measuring the relative amounts of these types of compounds (a property called the hydrophobic organic acid content), we determined that the DOC in tributaries draining more forested areas had a distinct make-up from DOC in tributaries draining agricultural areas.
The other two properties we analyzed were the relative amounts of different isotopes of carbon in DIC, DOC, and POC. The stable carbon isotope composition (i.e. the amount of carbon-13 versus carbon-12) is affected both by the organic source material and by biological processes such as photosynthesis and respiration. For example, the stable carbon isotope composition of POC can indicate whether the carbon is predominantly derived from land plants or from aquatic algae. Finally, the radiocarbon composition (the amount of carbon-14 versus carbon-12) is determined by the age of the carbon, meaning “old” carbon (like rock minerals or fossil fuels) looks very different from “young” carbon (like plants). So the radiocarbon composition of, for instance, fossil fuels (old) is very different from that of an ear of corn (young).
Sources Of Organic And Inorganic Carbon In The Mississippi River
Our study identified three main impacts of biological processes on the make-up of these forms of carbon in the Upper Mississippi River. First, we found that DOC carries a signature from various source materials that are not preserved in the other forms of carbon. The hydrophobic organic acid content and stable carbon isotope compositions in different tributaries tracked the amount of agriculture versus forest in the watersheds, while the radiocarbon composition of DOC showed that some tributaries reflect anthropogenic waste products like sewage and fossil fuels. Second, we found that at most times of the year, POC is primarily derived from organic material produced in the river itself, as opposed to material from land plants or soil. This shows that terrestrial organic material is consumed within the river, not just passively carried downstream.
Mississippi River transporting suspended sediment load into the Gulf of Mexico (Credit: NASA)
Finally, we used the stable carbon isotope composition of DIC to quantify the net effect of photosynthesis and respiration on the Upper Mississippi River. In essence, both photosynthesis and respiration are constantly consuming carbon dioxide and producing it; if photosynthesis is more active at any given time, there will be a net loss of carbon dioxide from the river and vice versa. We constructed an isotope “budget” to account for the effects of both physical processes (exchange of carbon dioxide with the atmosphere) and biological processes (photosynthesis) on DIC and found that the river is a net producer of carbon dioxide throughout the year – but not by a very large margin. Compared to the amount of carbon dioxide produced in other parts of the river in previous studies, our results suggest that the Upper Mississippi River is nearly balanced between photosynthesis and respiration, with a slight edge towards respiration.
This has interesting implications for our understanding of river ecology around the world. The Upper Mississippi River has been significantly altered from its pre-industrial state, with the widespread conversion of forests to agricultural lands and the construction of numerous navigation dams along its course. These dams create large “pools” in the river, which tend to be warmer and clearer than free-flowing stretches. These conditions can alter the balance of biological activity, for instance by allowing greater penetration of sunlight, which promotes photosynthesis. In the Upper Mississippi River, these dams appear to shift the ecosystem from a relatively more respiration-friendly environment to a more photosynthesis-friendly environment.
The Mississippi River isn’t alone. Many important industrial rivers, especially in Europe and North America, have experienced similar dam construction and land use change over the last decades and centuries. And as other countries seek to develop their rivers for commerce and flood control, many more navigation dams will likely be built around the world in the coming decades. These changes may have significant impacts on the chemical make-up and ecology of rivers globally like we observed in the Upper Mississippi.
This study, Biological and land use controls on the isotopic composition of aquatic carbon in the Upper Mississippi River Basin, was recently published in the journal Global Biogeochemical Cycles.